The process of the present invention relates to a "post-press" or second press operation for creating thick, hard, integral surface portions on at least one and preferably both surfaces of a man-made fiberboard or particle board substrate. A consolidated mat is first manufactured by consolidating a cellulosic fiber-containing composition to form a consolidated fiberboard or particle board. The consolidated mat can be consolidated by drying a mat formed by depositing a water slurry of fibers onto a water pervious support member, as is common in the manufacture of insulation board, or the mat can be consolidated under heat and pressure until substantially dry in a first press operation. The consolidated mat is thereafter "post-pressed" in a mold having the desired configuration to form the consolidated fiberboard into a desired shape, to impart surface texture if desired, and to form a surface skin to improve the stiffness, strength, paint holdout and design fidelity properties of the board. In accordance with one embodiment disclosed in our prior U.S. Pat. No. 4,175,150 a low density core material of less than 35 pounds/ft..sup.3 is maintained and a skin, formed in the after consolidation molding step with urea, is a higher density surface portion. In accordance with a second embodiment disclosed herein, heavier skinned surface products are formed in the same manner but starting with higher density consolidated mats on the order of 35 pounds/ft..sup.3 or greater, for example, 35-65 pounds/ft..sup.3. For the purpose of the present invention, a "post-press" operation refers to a press step performed on the mat after consolidation thereof.
The fiberboard of the present invention is generally formed in two separate operations. The first operation forms a condolidated mat while the second or "post-press" molding operation creates the surface skin portions. It should be noted however, that consolidated man-made boards previously manufactured are readily available and can be used as the consolidated mat to impart a skin to such consolidated mats in accordance with the post-press molding operation of the present invention. It is quite surprising that a "post-press molding" operation is effective in substantially altering the dimensions of the consolidated fiberboard and in providing a smooth surface skin on the consolidated fiberboard. Prior to the present invention, it was thought that the qualities and physical characteristics of a board would be finally determined by the conditions under which the board is initially consolidated and by the raw materials used to form the board, because of the bonding occuring during consolidation.
It is very difficult to both densify and restructure the surfaces of a consolidated fiberboard without destroying the fiber-to-fiber surface welds referred to in our co-pending application Ser. No. 739,184, filed Nov. 5, 1976, now U.S. Pat. No. 4,175,150. In accordance with the present invention, it has been found that the configuration, surface density, and physical characteristics of a completely consolidated fiberboard mat can be altered in a post-press molding operation when at least the surface fibers of the mat are contacted with urea prior to molding. During the post-press molding operation, at a temperature of at least 525.degree. F., the urea reacts to stiffen and strengthen the surface layers of a consolidated cellulosic fiberboard or particle board substrate to provide the strength necessary for the product to be useful as a furniture part while, at the same time, restructuring the board to a desired configuration. The resulting molded product has a look, feel and sound equivalent to that of natural wood, while being produced at a much lower cost.